The present invention relates to laser operation detection, and more particularly to an energy monitor and method for an F2 molecular fluorine laser system.
The present invention relates primarily to the field of industrial molecular fluorine (F2) lasers and the application of F2 lasers in optical lithography, annealing, micro machining, photo-ablation and others. Excimer lasers that are widely used today for these applications include XeCl lasers (308 nm), KrF lasers (248 nm), and ArF lasers (193 mn). However, in the near future, F2 lasers (157 nm) will be more widely used based on their very short wavelength, particularly for such applications as sub-0.18 micron photolithography. The short wavelength photon emission is advantageous for photolithography applications because the critical dimension (CD), which represents the smallest resolvable feature size producible using photolithography, is proportional to the wavelength. The smaller CD permits smaller and faster microprocessors and larger capacity DRAMs in a smaller package. The high 7.9 eV photon emission energy at this low wavelength is also readily absorbed in high band gap materials like quartz, synthetic quartz (SiO2), Teflon (PTFE), and silicone, among others, such that the F2 laser has great potential in a wide variety of material processing applications.
Significant improvements are being made in the development of the F2 laser to achieve an enhanced gain, longer pulse lengths, better pulse-to-pulse stability, and increased lifetime. Some of these improvements are described in U.S. patent application Ser. nos. 09/343,333, 09/317,526, 09/317,527, 09/317,695, and 60/140,530, each of which is assigned to the same assignee and is hereby incorporated by reference into the present application.
For many industrial and laboratory applications, excimer and molecular fluorine (F2) lasers are used in an operating mode that actively regulates and stabilizes the output power of the laser to a preset, configurable power or energy level. The active stabilization typically involves an energy detector that is connected to a control component for driving the high voltage that excites the F2 gas and for operating a gas control system. Accordingly, the driving voltage and gas injections/replenishments are actively adjusted to stabilize the output energy. This is possible because the output energy value depends on the input high voltage and the precise gas mixture in the laser tube. Thus, a variation of output energy may be compensated by adjusting the high voltage and/or laser gas mixture. See U.S. patent applications Ser. nos. 09/379,034, 60/123,928 and 60/124,785 (describing techniques for compensating output energy variations based on halogen depletion including gas replenishment, as well as high voltage adjustments over limited voltage ranges), each of which is assigned to the same assignee as the present invention and which is hereby incorporated by reference into the present application. Fast reliable energy monitors are very important basic modules for the required pulse energy control of the excimer lasers. Most applications of the 193 nm ArF and 248 KrF excimer lasers in optical lithography require energy dose control with high precision pulse-to-pulse regulation.
There are other parameters of the output beam of an excimer laser that require monitoring for various reasons. Among these output beam parameters are beam profile, bandwidth, wavelength, energy stability, pulse shape and pulse duration. In particular, it is desired to monitor any of these parameters in order to provide a feedback mechanism for controlling the laser during operation, particularly when the output beam is being used for precise industrial processing applications such as photolithography of small structures.
The UV laser radiation around 157 nm of the F2-molecule has been observed as being accompanied by further laser radiation output in the red region of the visible spectrum. This visible light originates from the excited fluorine atom (atomic transition). It is desired to have an F2-laser wherein the parameters of the UV (157 nm) portion of the output beam, and in particular the energy, may be monitored without substantial interference due to the accompanying red emission spectrum of the laser.
With the trend of providing lasers with lower and lower wavelengths, monitoring of the output energy, pulse length etc. is increasingly problematic, especially with those laser systems that include visible and/or infrared light accompanying the primary UV output. More specifically, silicon photodiodes have commonly been used as a reliable fast energy monitor for excimer lasers (sometimes with enhanced UV sensitivity or covered by phosphor coatings which down-convert the UV-radiation to the more sensitive visible spectral region of these Si-photodiodes). However, Si-photodiodes have a low spectral response to UV light, while having a significantly higher spectral response to visible and infrared light. Therefore any visible or infrared component in the laser output disproportionately affects the measured output value. This is especially problematic for the F2-laser, which includes red emission between 620 to 760 nm. Another problem with Si-photodiodes is that they degrade quite quickly when exposed to strong UV radiation, which is necessary to achieve a suitable detection level in this region of low detectivity.
Attempts have been made to selectively suppress certain wavelengths emitted from the discharge of the laser tube, especially the parasitic red/infrared laser light of the F2-laser. Because of the lack of suitable cut-off coatings or filters to separate the red radiation from UV on a single pass, a multi-pass beam delivery system has been designed. However, the beam path is very sensitive to mirror misalignment and degradation of the coatings of the many optical beam steering mirrors. Gratings have also been employed to separate red radiation from UV, but again with limited success.
Other types of detectors have been used for UV detection, with limited success. For example, vacuum photodiodes have been employed to measure the UV laser output. However, vacuum photodiodes tend to be cumbersome and expensive, and possess aging sensitive cathode layers. Even if special expensive materials are selected for coating the cathode to match the electron liberation energy to the quantum energy of the photons, they require a high vacuum and high voltage to operate, and therefore are not convenient for electronic signal processing on TTL voltage levels. Thermopiles can easily measure low UV output, but are not fast enough for single pulse detection at high repetition rates.
There is a need for a suitable fast energy detector that is capable of detecting single pulse energies at 157 nm without being adversely affected by associated red radiation, and which is fast enough to permit energy regulation at higher repetition rates without premature degradation due to UV exposure.
The present invention solves the aforementioned problems by providing an F2 laser system that utilizes a photo diamond detector that measures only the UV radiation in the laser beam. The photo diamond detector is fast enough to measure high pulse energies beyond 1 KHz, without premature degradation from the UV exposure.
The F2 laser system of the present invention includes a molecular fluorine (F2) gain medium disposed in a resonant cavity, a power supply for exciting the gain medium to produce a laser beam having an ultra violet (UV) radiation output at substantially 157 nm and a red radiation output in a 620 to 760 nm wavelength range, and a photo diamond detector that receives a portion of the laser beam for measuring at least one optical parameter of the UV radiation. The photo diamond detector is substantially insensitive to the red radiation output in the laser beam.
In another aspect of the present invention, a method of operating an F2 molecular fluorine laser system having a gain medium disposed in a resonant cavity comprises the steps of exciting a molecular fluorine gain medium to produce a laser beam having an ultra violet (UV) radiation output at substantially 157 nm and a red radiation output in a 620 to 760 nm wavelength range, directing a portion of the laser beam to a photo diamond detector, and measuring at least one optical parameter of the UV radiation using the photo diamond detector. The photo diamond detector is substantially insensitive to the red radiation output in the laser beam.
Other objects and features of the present invention will become apparent by a review of the specification, claims and appended figures.